CELL DIVISION (PPT PRESENTATION).pptx

CELL DIVISION: MITOSIS AND MEIOSIS CHROMOSOME: Chromosome are thread-like structures located in the nuclei of cells. Each chromosome is made up of one molecule of deoxyribonucleic Acid (DNA) coiled tightly many times around proteins called histones. Chromosomes are not visible in the nuc...

CELL DIVISION: MITOSIS AND MEIOSIS CHROMOSOME: Chromosome are thread-like structures located in the nuclei of cells. Each chromosome is made up of one molecule of deoxyribonucleic Acid (DNA) coiled tightly many times around proteins called histones. Chromosomes are not visible in the nucleus of cells, even under the microscope, but during cell division the DNA of the chromosome becomes condensed (more tightly packed) and is then visible under the microscope. STRUCTURE OF CHROMOSOME (Adapted from U.S. National library of Medicine) figure 1 Structure of Chromosome continue Each chromosome has a constriction point called the Centromere that hold together two identical chromatids. The centromere divides the chromosome into two sections – the shorter arm is known as ‘’petit’’ or P – arm and the longer harm called the q – arm Based on the position of the centromere, chromosomes are grouped into Metacentric, submetacentric and acrocentric Metacentric chromosomes- the position of the centromere is ½ the length of the chromosome Submetacentric – when the position of the centromere is 1/3 the length of the chromosome Structure of Chromosome continues Acrocentric – Centromere is positioned at the end of the chromosome with no p- arm Telomere is a specialised sequence of DNA attached at the terminal end of chromosome. It seals the chromosome end to prevent fusion with other chromosomes The centromere in addition to holding the chromatids together, also provide site for anchorage for the spindle apparatus Chromosome continues As the genetic material passes from the parents to the offspring during reproduction, the chromosomes are responsible for containing the instructions (genes) that make the offspring unique while still carrying traits from the parents. Humans have between 20,000 t0 25,000 genes on 46 chromosomes which appear as 23 homologous pairs. There are 22 pairs of autosomes in somatic cells and 1 pair of sex chromosome. Each set of 23 chromosomes is called a haploid set and when a cell has 2 complete sets, it is described as diploid A karyotype is a display of chromosomes ordered from 1 – 22 pairs plus a sex chromosome with each chromosome oriented so that the p – arm is on top. Females have 46XX karyotype and males have 46XY karyotype A sample of a Karyotype ( Adapted from Campbell Biology)  MITOSIS AND MEIOSIS Cell division is a process by which a parent cell divides into two or more daughter cells. There are two distinct types of cell division – Mitosis and Meiosis, but with several common features. Between two successive cell division is a period of rest known as interphase. During the interphase preceding cell division, the DNA of each chromosome is duplicated in both mitosis and meiosis. Thus, each chromosome has two identical chromatids joined at the centromere. See figure 2 in next slide Figure 2. A cell in Interphase stage of cell division (Adapted from Campbell Biology)  Mitosis Mitosis is a process by which a parent cell divides to produce two daughter cells that are genetically identical to each other and to the parent cell The process consist of six stages – Prophase, Pro-metaphase, Metaphase, Anaphase, Telophase and cytokinesis 1. Prophase: Two identical chromatids start to shortened and thickened (condensed) and become barely visible as chromosomes within the nucleus. In addition, a spindle apparatus of tubulin fibres start to assemble 2. Pro-metaphase: The nuclear membrane dissociates, centriole divides and each half begins to move to each pole, tubulin fibres enter the nucleus and attached at the kinetochore that has formed around the centromere. The chromosomes are more distinguishable Diagram of prophase stage of mitosis (Adapted from Campbell Biology) Diagram of prometaphase stage of mitosis (Adapted from Campbell Biology) Mitosis continue 3. Metaphase: The chromosomes are at their most condensed state and tension in the spindle fibres cause the chromosomes to line up along the metaphase plate 4. Anaphase: Sister chromatids now begin to separate and are drawn to opposite poles by the tubulin fibres 5. Telophase: Nuclear membrane begins to assemble around the separated chromatids (chromosome) as they arrived at the spindle pole. Chromatin starts to decondensed and become more diffused 6. Cytokinesis: Cleavage of the cell membrane occurs to form two separate daughter cells. The daughter cells are genetically identical to the parent cell. Metaphase stage of mitosis (Adapted from Campbell Biology) Anaphase and Telophase/Cytokinesis stage of mitosis (Adapted from Campbell Biology)  MEIOSIS This is the type of cell division that takes place in the germ cells to produce male and female gametes (Sperm and Ovum) Meiosis consist of two successive rounds of cell division (meiosis I and meiosis II) and result to the formation of four daughter cells each containing 23 chromosomes that are genetically different Just like mitosis, the process of meiosis is preceded by Interphase. This is followed by meiosis I (Reduction division) which has 4 stages – prophase I, metaphase I, Anaphase I and Telophase I Diagram showing some key terms in Meiosis (Adapted from Bradley Schaefer) Meiosis I continues Prophase I of first meiosis is lengthy and is subdivided into several sequential steps: 1. Leptotene – the chromosomes becomes barely visible as long thin structures 2. Zygotene – homologous pairs of chromosomes from each haploid set come to lie side by side along parts of their length forming tetrads 3. Pachytene - the chromosomes start to shorten and thicken and become more closely contacted in pairs along their entire length. During this time synapsis, crossing over and chromatids exchange take place and the nucleoli disappear 4. Diplotene/Diakinesis – the chromosomes becomes Diagram showing stages of prophase 1 (Adapted from Bradley Schaefer)  Meiosis I continues The paired homologous chromosomes show evidence of crossing-over and chromatid exchange and display characteristic Chiasmata Metaphase I: the nuclear membrane breaks down and homologous pairs of chromosomes are aligned on the equator of the spindle apparatus Anaphase I: homologous chromosomes move in opposite direction toward the two poles Telophase I: cytokinesis occur. Nuclear membrane may temporarily be re-formed, yielding two daughter cells, each with half the number of chromosomes (haploid cells), but each chromosome consisting of two genetically unique chromatids  Meiosis II In meiosis II, there is no replication of DNA, but another round of prophase, pro-metaphase, metaphase, anaphase, telophase and cytokinesis takes place. These stages occur much in the same way as in mitosis The end result of meiosis II is the production of four (4) haploid daughter cells, each containing 23 1n chromosomes Unlike the cells in mitosis, these daughter cells are genetically unique and different from the parent cells  Meiosis continue The sequence of events during Spermatogenesis and Oogenesis is basically the same. However, few differences occur. In males, the process of meiosis is initiated at puberty (onset of spermatogenesis) and completed without arrest. The 4 spermatids formed after meiosis undergo cytodifferentiation to eventually become Spermatozoa In the females, meiosis is initiated in the foetus (around 12 weeks), Primary oocytes are arrested at the diplotene stage of prophase I until LH surge at ovulation initiates metaphase I. After completion of meiosis I, the primary oocyte becomes secondary oocyte in which one nucleus forms the first polar body that degenerates without further division. Meiosis continues and is arrested again at metaphase II until it is fertilized by a sperm cell when the final meiotic division is completed SIMILARITIES BETWEEN MITOSIS AND MEIOSIS In both mitosis and meiosis, there is need to duplicate the entire chromosome content prior to cell division Both types uses the cell machinery of the parent cell to make the DNA, RNA and new proteins that will participate in the cell division Both processes rely on using the mitotic spindle to separate the chromosomes into the two poles of the cell Major differences between mitosis and meiosis MITOSIS MEIOSIS 1. Occurs in somatic cells Occurs in germ cells 2. Takes about an hour to complete the In females, it begins at 12 weeks process and no difference b/w sexes gestation and arrest at 20 wks ( at in respect to age of commencement diplotene of prophase 1). 1st meiotic division is completed at ovulation and 2nd meiotic division is completed at 3. Chromosomes do not pair fertilization. Males begin at puberty 4. Usually, no recombination takes and is not arrested place Homologous chromosomes pair 5. No change in chromosome number Recombination always takes place Chromosome number always reduced to one of each pair (haploid set in each 6. One round of DNA replication; one daughter cell, n = 23 cell division produces two identical One round of DNA replication, two cells daughter cells divisions produce four genetically 7. No change in gene content different daughter cells. All daughter cells will be genetically different, due to segregation of